This invention relates to nuclear magnetic resonance (NMR) imaging and, more particularly, to methods for overcoming chemical shift artifacts in NMR imaging by imaging with selected nuclei of a sample containing nuclei of the same, or different, species having chemically-shifted NMR frequencies.
As is well known, the nuclear magnetic resonance phenomenon is exhibited by atomic nuclei with an odd number of either protons or neutrons. Such nuclei possess spin, which endows them with a small magnetic field. When placed in an externally applied static main magnetic field, B.sub.o, the nuclei tend to align themselves with the applied field and produce a net magnetization, M, in the direction of the applied field. The nuclei oscillate or precess about the axis of the applied field with a characteristic NMR frequency, .omega., given by the Larmor equation: EQU .omega.=.gamma.B.sub.o ( 1)
where .gamma. is the gyromagnetic ratio and is constant for each NMR isotope. If a time-dependent (RF) magnetic field, having frequency components equal to the Larmor frequencies of the nuclei, is applied in a direction orthogonal to the main field, then the nuclei will absorb energy and nutate away from the axis of the main field and commence to precess at the Larmor frequency about the new net applied field direction. If the RF pulse is turned off precisely when the angle of nutation reaches 90.degree., the magnetization is left in the transverse, or x-y, plane and the net magnetization now precesses about B.sub.o in the transverse plane at the Larmor frequency. Such a pulse is termed a 90.degree. pulse. A 180.degree. pulse is one which nutates the magnetization through 180.degree., inverting it. These two types of RF pulse form the basic tools of the NMR spectroscopist.
Experimentally, the NMR signal is detected by a tuned RF coil with axis perpendicular to B.sub.o. The same coil used for excitation is also suitable for detection, or alternatively, a separate, mutually orthogonal, coil can be used. The oscillating NMR magnetization induces a voltage in the coil, analogous to the principle of an electric generator. The induced signal immediately following an RF pulse is termed a free induction decay (FID), reflecting the decay in the time signal as nuclei relax back to equilibrium in alignment with the main field or which signal decays due to dephasing caused by inhomogeneities in the main field. These NMR signals may be detected and Fourier-transformed to derive the frequency components of the NMR signals characteristic of the excited nuclei.
Nuclei of the same isotope can exhibit minute variations in their NMR frequencies, which are referred to as chemical shifts, because of differences in their chemical environments which cause differences in their local magnetic field environments. Chemical shifts result from alterations of the magnetic field around nuclei as a result of the shielding currents that are associated with the distribution of electrons around adjacent atoms. The degree of shielding is characteristic of the environment of the nucleus, and thus the chemical shift spectrum of a given molecule is unique and can be used for identification. In conventional NMR spectroscopy, chemically-shifted signals are observed from the whole of an NMR sample for studying the chemical structure of the sample. Because the resonant frequency and the absolute chemical shift are dependent upon the strength of the field, the chemical shift is expressed as a fractional shift in parts-per-million (ppm) of the resonant frequency relative to an arbitrary reference compound.
Since the Larmor frequency is proportional to the magnetic field, if the magnetic field varies spatially in a sample, then so does the resonant frequency of the nuclei. In NMR imaging, at least one magnetic field gradient is applied to the sample to spatially encode the emitted NMR signals. If, in the presence of gradients, an RF excitation pulse having a narrow range of frequency components is applied to the nuclei in a selected region, e.g., a slice or a selected point of the sample, this region is selectively excited and the NMR signals from the selected region can be detected. The data collected from different regions or points of the sample can be processed in a well-known manner to construct an image.
NMR imaging in the past has typically been performed in rather low magnetic fields and chemical shifts have not been a significant problem. In magnetic fields below about 0.5 T, chemical shifts are difficult to observe because the chemical shifts are comparable to the natural linewidths of the resonances and the low sensitivity of nuclei other than hydrogen (.sup.1 H). It is desirable, however, to perform NMR imaging in higher magnetic fields, e.g. in fields in excess of 1 T, because of the improved signal-to-noise ratios realized; recent advances in magnet technology permit the use of higher magnetic fields of the order of 1-1.5 T in medical and biological NMR imaging. As the magnetic field increases, the chemical shift increases proportionately and becomes a greater problem. Chemical shift can produce the same effect as a spatial variation in the NMR signal. This results in chemical shift artifacts which are manifested as rings in multiple-angle-projection imaging and ghosts in two-dimensional-Fourier-transform (2DFT) imaging. Ghost artifacts, for example, may appear as a faint ring or ghost at one side of an image, and they obliterate some of the spatial information present. In proton imaging of the body, the chemical shift observed is principally between the hydrogens attached to oxygen in water (H.sub.2 O) and the hydrogens attached to carbon in alkyl --CH.sub.2 -- groups found in lipid (fat) and other tissues. The effect of the chemical shift is to produce two superimposed images, with one image being the water image and the other image being the lipid image shifted by an amount corresponding to the chemical shift.
The observation and recognition of chemical shift artifacts in NMR imaging was first published by the present inventor, Paul A. Bottomley, in "A Versatile Magnetic Field Gradient Control System For NMR Imaging", J. Phys. E: Sci. Instrum., Vol. 14, 1981, where the expression: ##EQU1## is proposed for the minimum imaging gradient, g, required to resolve N pixels of a sample of spatial extent, a, containing a spectrum of chemical shifts, .delta.*, measured in frequency units to avoid chemical shift artifact. Since the chemical shift range in frequency units increases linearly with magnetic field strength and it is desirable to increase N to maximize spatial resolution, a condition is rapidly approached where practical gradient strengths are insufficient to satisfy equation (2). Furthermore, it is disadvantageous to increase the gradient strength beyond what is absolutely necessary due to inherent main field inhomogeneity, because this increases the frequency bandwidth of the NMR signal and therefore reduces the signal-to-noise ratio.
In general, it is not possible to correct for chemical shift artifacts from a single NMR scan by calculation unless prior knowledge is possessed of either the spatial information or the chemical shift spectral information, including the amplitudes of the peaks. Such knowledge would be contrary to the aim of an imaging experiment, which is to investigate the interior of an unknown object. Thus, chemical shift artifacts are a significant problem, particularly for high field imaging in homogeneous main magnetic fields.
It is therefore desirable to provide NMR imaging methods that overcome chemical shift artifacts. It is also desirable to provide NMR imaging methods that enable resolution of chemically-shifted species. For example, an image constructed from CH.sub.2 lipid alone may prove useful for looking at fat or atheroscelerotic lesions or plaques in blood vessels, as well as for the evaluation of heart disease.